Alzheimer's disease (AD) is the most common cause of dementia in the elderly. In the U.S., this disease affects approximately 3-4 million persons, costing the U.S. economy more than $50 billion per year. The cause(s) of this debilitating neurodegenerative disease is/are presently unknown. However, a large body of evidence indicates that at least some, if not all, AD cases are due to genetic factors. Genetic analysis of families with multiple cases of early-onset AD has shown that 3 autosomal-dominant genes are responsible for at least some occurrences of the disease. In these families, offspring of affected persons are at least at 50% risk of inheriting a Familial AD (FAD) gene and developing AD. Late-Onset FAD (LOFAD) appears to involve other genes, and is a more complex disease. Using linkage analysis, other sophisticated statistical genetic methods and positional cloning approaches, the long-range goal of this project is to identify the underlying causes of AD by identifying the genes responsible for genetic forms of late-onset AD. Using genetic-linkage analysis, based on Monte Carlo Markov Chain methods, we identified a quantitative trait locus on chromosome-19p 13.2 that affects AD risk. This locus was identified as a quantitative trait that affects age-of-onset. The 19p locus targeted by this project is distinct from ApoE, another LOFAD gene located at 19q13. To identify this new LOFAD gene by positional cloning, the following steps will be performed. First, a physical, sequence, and gene-map of 19p13.2 spanning the region, indicated by linkage analysis, will be generated. Second, genes in this region will be screened for polymorphic sites by database analysis and DNA sequence analysis. Third, polymorphisms spanning the region will be used to test for linkage disequilibrium in the region. Polymorphic sites tested will include short tandem repeat polymorphic sites and single nucleotide polymorphism (SNP) sites. Fourth, SNP's in genes in the region will be tested as pathogenic sites in multiple familial and case-control samples to identify the true pathogenic allele. Fifth, when the gene and pathogenic alleles are found, functional assays will be devised to determine the mechanism of pathogenesis leading to AD. Identification of additional LOFAD genes should greatly enhance our understanding of AD, and potentially lead to new types of therapies.